Flash memory systems are well-known. In typical flash memory systems, a sense amplifier is used to read data from a flash memory cell. FIG. 1 depicts a prior art sense amplifier 100. Sense amplifier 100 comprises selected flash memory cell 102, which is the cell to be read. Sense amplifier 100 also comprises reference flash memory cell 122, against which selected flash memory cell 102 is compared. PMOS transistors 104, 106, 124, and 126 and NMOS transistors 108, 110, 112, 128, and 130 are arranged as shown. PMOS transistor 104 is controlled by CASREF (column address strobe reference), PMOS 106 is controlled by SEN_B (sense amplifier enable, active low), NMOS transistors 108, 112, and 128 are controlled by ATD (address transition detection, which detects a change in the received address), and NMOS transistors 110 and 130 are controlled by YMUX (Y multiplexor) which activates a BL (bit line). Selected flash memory cell 102 receives WL (word line) and SL (source line), and reference memory cell 122 receives SL (source line). Comparator 130 receives two inputs that are directly related to the current drawn by selected flash memory cell 102 and reference memory cell 122, and the output SOUT is directly indicative of the data value stored in selected flash memory cell 102.
One drawback of prior art sense amplifier 100 is that a constant current is drawn by reference memory cell 122 and its associated circuitry, which results in significant power consumption. In addition, reference memory cell 122 and its associated circuitry typically are provided in a separate read bank than the read bank in which selected memory cell 102 is located, which requires a large die area and more power consumption for additional Y-decoding. Also, the CASREF signal also is sensitive to noise, and the CASREF circuit also consumes significant standby current.
What is needed is an improved sense amplifier design for a flash memory system that consumes less power than prior art sense amplifier solutions. What is further needed is an embodiment of a sense amplifier that does not require a separate read bank of memory cells. What is further needed is a sense amplifier that can accurately detect small differences in current drawn by selected flash memory cell 102 and reference memory cell 122, as might be required during a Margin0/1 mode.